1. Field of the Invention
The invention relates generally to compositions that may be useful as fuel cell catalysts.
2. Description of the Prior Art
Proton exchange membrane fuel cells (PEMFCs) are currently under intense development as high-efficiency energy conversion devices. Before they become commercially viable, though, the cost of PEMFCs must be significantly reduced. A major contributor to the high cost of the fuel cells is their platinum catalysts, which are used to oxidize hydrogen and reduce oxygen at the anode and cathode respectively. The oxygen reduction reaction (ORR) and hydrogen oxidation reaction (HOR) are given in Eqs. 1 and 2. The platinum catalysts lower the activation energy of the reactions and make the PEMFCs efficient.O2+4H++4e−→2H2O  (1)H2→2H++2e−  (2)Because the ORR is a 4-electron reaction, it is kinetically limited. To overcome this limitation, high platinum loadings at the cathode (e.g., 0.2 mg Pt/cm2) have been used. Reducing platinum loading by at least a factor of 10 would help to make PEMFCs cost effective.
Researchers recognized years ago that the Pt content of PEMFC electrodes could be reduced by dispersing nanoscale Pt particles on a porous, electronically conductive media (Vulcan carbon) and adding a proton conducting media (a perfluorosulfonic ionomer, Nafion®) (Raistrick, U.S. Pat. No. 4,876,115. All referenced patents and publications are incorporated by reference). When surrounded by Vulcan carbon and Nafion, the Pt serves more effectively as an electrocatalyst for hydrogen oxidation and oxygen reduction because there are ample transport paths for protons and electrons. Whereas the catalytic activity of the Pt is limiting, the electrode reactions are mediated by the rate of the transport of the gases, protons, electrons, and water to and from the Pt surfaces.
A few other reports have tried to improve the activity of Pt by dispersing it on oxide supports. Tseung and Dhara disclosed a dispersion of metallic Pt on a semiconducting oxide support (Tseung et al., “The reduction of oxygen on platinised Sb doped SnO2 in 85% phosphoric acid,” Electrochim. Acta, 1974, 19, 845-848.). Antimony-doped tin hydroxides were prepared in solution and then sintered at 500° C. to ensure good electronic conductivity, and then the oxides were impregnated with Pt and reduced in hydrogen. The Sn-based catalysts performed well vs. Pt blacks during pulsed measurements, but the steady-state performance of the Sn catalyst was poor.
Watanabe et al., “Preparation of dispersed platinum on conductive tin oxide and its catalytic activity for oxygen reduction,” J. Electrochem. Soc., 1998, 145, 3713 disclosed the preparation of anhydrous platinum tin oxide from an aqueous solution by spraying an aqueous solution of Sn onto a Pyrex surface held at 450° C. to make anhydrous SnO2 thin films. The SnO2 was then soaked in base and then treated with chloroplatinic acid. The materials were tested in half cells in alkaline solution for their ORR activity. Materials heated over 200° C. were most active, but the materials were not stable over long term use.
Another form of a Pt—SnOx catalyst was evaluated for its activity for methanol oxidation at a fuel cell anode (Katayama, “Electrooxidation of methanol on a platinum-tin oxide catalyst,” J. Phys. Chem., 1980, 84, 376-381). Pt on Sb-doped SnOx was prepared by spraying mixtures of tin, antimony, and platinum chlorides onto glass at 550-600° C. The catalysts were initially active, but were reduced over time in methanol, and lost their activity.
Pt—SnOx catalysts have also been developed for the oxidation of trace CO in CO2 lasers (Gardner et al., Proceedings of NASA Conferences on Long-Life CO2 Laser Technology, 1986, 1989, 1991, and 1992). The Pt—SnOx was typically dispersed on a silica support and heated. The catalysts heated at 150° C. had superior properties superior to those heated at 250° C. (Gardner et al., “Characterization study of silica-supported platinized tin oxide catalysts used for low-temperature CO oxidation: effect of pretreatment temperature,” J. Phys. Chem., 1991, 95, 835-838.). The active catalyst was attributed to sub oxides and tin metal.